The invention relates to a technique for mounting a semiconductor device.
There is an increasing demand for semiconductor devices such as BGA, CSP, WPP, flip chip and etc. each of which semiconductor devices has bumps as external pads. In the semiconductor device having the bumps as the external pads, the bumps are formed by, for example, the steps of: preparing a pad pattern in which the plating of nickel-gold (Ni/Au) and nickel-palladium-gold (Ni/Pd/Au) is applied on the surface of copper; coating the pad pattern with flux by printing; mounting solder balls on the flux-coated pad pattern; and reflow-heating them so that the bumps may be formed.
Further, the mounting of these semiconductor devices is performed by the steps of: coating a paste material, which comprises solder particles and flux, on a pad pattern formed on a substrate; positioning the bumps of the semiconductor device and the pad pattern of the substrate; loading the positioned bumps on the pad pattern; and soldering them.
Usually, a material used for the bumps or the mounting is so-called Sn—Pb eutectic solder of Sn-38 mass % Pb.
In recent years, it is urged to put Pb-free solder to practical use.
The melting point of the conventional Sn—Pb eutectic solder is 183° C. On the other hand, in a Pb-free solder material such as, for example, An—Ag—Cu solder, the melting point thereof ranges from 216° C. to 227° C., which melting point is higher than that of the conventional Sn—Pb eutectic solder.
According to the research performed by the inventors of the invention, it is found that, in a case where a semiconductor device such as BGA which device has bumps is mounted on a substrate, there occurs such a phenomenon as the solder bumps do not always melt readily. Namely, according to the inventors' research, it is found that a temperature between the semiconductor device and the substrate is lower than the temperature of the substrate or the semiconductor device itself by a value of 15 to 20° C.
Thus, by setting the reflow temperature so that these Pb-free solder bumps may be melted, the temperature of the substrate or the semiconductor device itself becomes higher by the value of 15 to 20° C. than the temperature measured between the semiconductor device and the substrate, that is, it becomes 240 to 260° C., so that there arises a problem of heat resisting property regarding the substrate and other electric parts (such as electrolytic capacitors and etc.) mounted on the substrate.